1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of manufacturing the same.
2. Description of the Related Art
The principles of the display operation of a typical liquid crystal display device are explained hereinafter. A liquid crystal display device includes two substrates disposed opposite to each other with a liquid crystal layer interposed therebetween. A pixel electrode and a common electrode are formed in an area that corresponds to each pixel. The alignment state of liquid crystal molecules is controlled by voltage applied between these electrodes. A polarizing plate is disposed on each of the incoming light side and the outgoing light side of the pair of two opposed substrates. The light transmittance is controlled by changing the alignment state of the liquid crystal molecules such that a desired image is displayed.
There are generally two categories for the driving methods of liquid crystal display devices. They are called a vertical electric field mode and a horizontal electric field mode respectively. In a vertical electric field mode liquid crystal display device, a pixel electrode composed of a transparent electrode is formed in an area corresponding to each pixel in one of the two substrates that are opposed to each other with a liquid crystal layer interposed therebetween, and a common electrode composed of a transparent electrode is formed in the other one of the two substrates.
In a horizontal electric field mode liquid crystal display device, a pixel electrode and a common electrode are formed in an area corresponding to each pixel on the surface of the liquid crystal layer side of either or both of the two substrates that are opposed to each other with a liquid crystal layer interposed therebetween. The application of voltage between the pixel electrode and the common electrode produces an electric field having a component roughly parallel to the substrate. The generated electric filed having the component roughly parallel to the substrate changes the alignment state of the liquid crystal molecules, so that the light transmittance is controlled and a desired image is displayed.
In recent years, liquid crystal display devices have been used in a wide range of applications such as TVs, handheld data terminals, and monitors for personal computers. As a result, they may come under the influence of various external environments. Especially, when the surface is electrically charged by static electricity or the like, it may adversely affect the alignment state of the liquid crystal through the electrode substrate, or may even destroy the TFTs (Thin Film Transistors). Therefore, when a liquid crystal display device is electrically charged, it needs to immediately remove the charged electricity. Especially in the case of a horizontal electric field liquid crystal display device, the effect on the liquid crystal alignment state is significant and the display quality is remarkably deteriorated since no electrode is formed on one of the substrates for driving the liquid crystal.
To solve these problems, Japanese Unexamined Patent Application Publication Nos. 9-105918, 10-96953, and 10-293207 disclose a method in which a transparent conductive film is formed on the back surface of the substrate and electrically connected to a ground. This structure can reduce effects to the alignment of the liquid crystal and to the thin-film transistors caused by static electricity generated on the surface of the liquid crystal display device.
Conventionally, a color filter substrate having a transparent conductive film such as an ITO film formed on its back surface (the surface opposite to the surface on which the film of color material or the like is formed) has been used. In recent years, the demand for thinner and lighter liquid crystal display devices has grown, and therefore the substrates need to be thinned. Because of the restrictions on manufacturing equipment and manufacturing processes, substrates having a thickness thinner than a certain thickness causes problems in conveyance or a similar process due to the bending of the substrates, and therefore cannot be manufactured. Therefore, substrates need to be thinned by grinding or etching after the color filter substrates and the TFT substrates are superimposed with each other. Accordingly, a color filter substrate on which the transparent conductive film is formed in advance cannot be used.
As described above, the transparent conductive film needs to be formed after the thinning process of the substrate. A typical method of forming a transparent conductive film is a sputtering method, which is conventionally used to form an ITO film. However, the space between the superimposed substrates is hermetically sealed by sealing the peripheral portion thereof with resin or the like in order to prevent the infiltration of water during the thinning process of the substrates. Therefore, if the transparent conductive film is formed in a vacuum like the case of a sputtering method, the substrates may be cracked due to the pressure difference between the inside and outside of the substrates. Accordingly, since vacuum film formation cannot be used as a proper method of forming a conductive film for thin substrates, an application method such as a spin coat method has been examined.
In recent years, the demand for improving the performance of liquid crystal display devices has been increasingly growing. Therefore, even the reduction in the transmittance due to the formation of the transparent conductive film has been regarded as a problem. To minimize the reduction in the transmittance, the film thickness of the transparent conductive film needs to be thinned. However, the resistance of the transparent conductive film needs to be reduced in order to reduce the effect of static electricity. Accordingly, the transmittance and the resistance of the transparent conductive film behave in a mutually contradictory manner with respect to the film thickness. The optimal values for them are not clearly established until now, and conductive films having a resistance larger than necessary have been used in the past. Therefore, the transmittance has been sacrificed, and therefore the display performance of liquid crystal display devices has not been able to be fully exploited.
In addition, a formation method of a transparent conductive film where the process is carried out in a vacuum, such as a sputtering method, cannot be applied to reduce the thickness of the substrate, because it may causes a crack in the substrate as described above. Therefore, the transparent conductive film needs to be formed by using a wet method such as a spin coat method. However, a transparent conductive film formed by the wet method has not been able to be applied since it has a higher resistance in comparison with an ITO film or the like.
The present invention has been made in view of such circumstances. One of the objects of the present invention is to provide a liquid crystal display device that is capable of suppressing the effect of static electricity and has improved display performance by optimizing the thickness of the transparent conductive film, and a method of manufacturing the same.